Method for producing torrefied wood, product obtained thereby, and application to the production of energy

ABSTRACT

New product consisting of wood which is torrefied between 250° and 280° C. in a non oxidizing atmosphere, in the form of sticks of uniform length: 15 mm for example and having a diameter comprised between 5 and 20 mm, which are not disbarked. The preparation of the method comprised the obtention by culture of rectilinear ligneous rejections, the cutting, drying and torrefaction thereof preferably in a vertical reactor (101) where the material to be torrefied (115) is traversed by a gas stream circulating at high speed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a product originating from thetorrefaction of pieces of wood, to a process for the production of thissubstance and to a particular application of this product.

2. The Prior Art

It is known that wood which is heated to a temperature of approximately200° to 300° C. becomes what may be called "torrefied wood", i.e., aproduct with a high fixed carbon content, a high calorific value, a verylow moisture content, (approximately 3%), and which is non-hygroscopicand rot-proof, having retained practically all the pyroligneouscompounds.

It has been proposed to bring woody pieces of different origins to thestate of torrefied wood in order to agglomerate them using an addedbinder (FR-A-953,004) or using the tars produced by the wood itself asthe binder (CH-A-228,877), to produce a fuel therefrom.

It has also been proposed (EP-A-007,374) to use wood torrefied under aneutral atmosphere without subsequent agglomeration, especially as fuel.

A drawback of torrefied wood, as proposed until now, is that it isproduced from waste from forest plantations or from the wood industry,i.e., from materials of very irregular shape and composition. A productwhich itself is irregular, and which has a variable particle size andespecially a wide particle size range results therefrom. Because ofthis, a product which does not lend itself well to automatic treatmentis obtained, due to the fact that it is subject to blockages or vaultformations during its movement, and that its permeability, in bulk, isvery irregular. These defects may be overcome for example by cuttingpieces of wood to uniform size, or by agglomerating fine particles,especially sawdust, but these operations are expensive, and thus reducethe economic advantage of torrefied wood.

The object of the invention is to provide a process for the productionof thermal energy from torrefied wood which is more advantageous fromthe point of view of its regularity and its cost than similar processescurrently known.

Another object of the invention is to provide, by way of new products,torrefied wood obtained as intermediate in the above process, theprocess of its production itself comprising the first stages of thisprocess of thermal energy production. It is clear that the fact of beingable to provide the product for other uses is an additional cause forthe lowering of energy cost, due to a better use of the investment.

SUMMARY OF THE INVENTION

Consequently, the invention provides a process for the production ofthermal energy, comprising the torrefaction of pieces of wood at atemperature of 250° to 280° C. in a non-oxidizing atmosphere, followedby a cooling and a combustion of the torrefied pieces of wood, definedin that it comprises the following stages:

(a) cultivation of crops capable of giving, after cutting, substantiallystraight woody suckers of diameter between 5 and 20 mm, the cultivationof the crops being carried out so as to obtain the suckers;

(b) harvesting the suckers, followed by a sectioning of the latter intounbarked "green" pieces of stick, of uniform length, of betweenapproximately 10 and 25 mm;

(c) thermal treatment of the green pieces of stick comprising a dryingfollowed by a torrefaction between 250° and 280° C.;

(d) cooling; and

(e) combustion of the torrefied product in a plant of automatedoperation.

The invention also provides a process for the production of the productwhich can be used especially for thermal energy production, this processcomprising the torrefaction of pieces of wood at a temperature of 250°to 280° C. in a non-oxidizing atmosphere; followed by a cooling, definedin that it comprises the following stages:

(a) cultivation of crops capable of giving, after cutting, substantiallystraight woody suckers of diameter between 5 and 20 mm; the cultivationof the crops being carried out so as to obtain the suckers;

(b) harvesting the suckers, followed by a sectioning of the latter intounbarked "green" pieces of stick, of uniform length, of betweenapproximately 10 and 25 mm;

(c) thermal treatment of the green pieces of stick comprising a dryingfollowed by a torrefaction between 250° and 280° C.; and

(d) cooling.

The invention also provides a new product originating from thetorrefaction between 250° and 280° C. of woody fragments, defined inthat it is essentially in the form of small pieces of stick of diameterbetween 5 and 20 mm; which are unbarked and of uniform length of betweenapproximately 10 and 25 mm.

The sectioning operations provide green pieces of stick of uniformlength and which are more or less equal in their mean diameter, which isthe most favorable configuration for subsequent automatic treatmentsbecause this configuration leads to more regular flows in bulk, withvery constant natural slopes and a very high and also uniformpermeability in bulk.

Preferably, the cultivation comprises the production of rooted stumps ofthe crops and periodic cuttings of the suckers above the level of thecollar of each plant, the successive cuttings being carried out atincreasing heights, at least until the density of suckers per unitsurface area reaches the desired value.

It is understood that it is an "industrial" type of cultivation, muchcloser to cereals than to conventional forest plantations, and that itcan easily be mechanized, with equipment similar to, and sometimes thesame as, those used in cereal farming.

Advantageously, the cutting is carried out when the suckers are 1.5 to 2meters in height, and preferably after leaf drop. A higher uniformity inthe shape of the sections is thereby obtained, as the diameter of thelong suckers varies slowly from the base to the terminal bud, and theabsence of leaves removes the need for a subsequent grading,facilitating the positioning on an automatic sectioning machine at thesame time.

Advantageously, the sectioning is carried out immediately after cutting.All transport and handling problems are thereby avoided.

According to an advantageous embodiment, the sectioning is carried outby aligning the cut suckers parallel to one another, and by passing thembefore a series of rotary blades. Sections of equal length are therebyobtained, with simple machines.

An important step in the process is the torrefaction. In effect, beyonda temperature of the order of 250° C., the conversion of the woodbecomes exothermic, and, in the presence of air, if the reaction is notwell controlled, there is a risk of catching fire. It is therefore knownthat the use of a neutral atmosphere is preferable.

According to a preferred embodiment of the invention, use is made of thefact that the "green" sections just like the torrefied sections, have,in bulk, a high and regular gas permeability, although each individualsection is not permeable. A torrefaction method defined as follows isused: a mass of sections to be torrefied is placed in a reactor, anon-oxidizing gas is circulated through this mass, this gas penetratinginto the mass at a temperature greater than that of torrefaction,passing through the mass by first passing over the undried or partiallydried sections, and leaving the mass by passing over the sectionsnearing completion of torrefaction and the gas circulation speed is onlyslightly lower than that which would cause the torrefied sections to becarried away by the stream of gas.

The procedure of co-current circulation of gases results firstly in asudden heating of the green sections, with a quick removal of waterwhich leads to a large pore opening, and secondly to only a smalldifference in temperature between the sections nearing completion oftorrefaction and the gas surrounding them, which makes it easier tocontrol the reaction, especially with a view to preventing it fromoverspeeding. However, the gases circulate at high speed, whichfacilitates the achievement of homogeneous temperatures. This speed isfacilitated by the permeability of the mass.

Preferably, the non-oxidizing gas circulates in part in a closedcircuit, and essentially consists of water vapor originating from thesections and combustion gas coming from a burner intended for increasingthe temperature of the gas. The high thermal capacity of the steamfacilitates the exchanges; moreover, this steam has the advantage ofbeing free in this case. Additionally, it is neither toxic norpolluting.

Advantageously, a part of the said gas is cooled and a part of its waterremoved, and it is then passed through the mass which has just beentorrefied, in order to cool it and to prevent the initiation ofexothermic reactions. A particularly valuable system is thereby formed,as the thermal capacity of the steam is once again used for theregulation of the satisfactory operation of the process.

The above embodiments enable very short operating times to be used, andadvantageously, the time for torrefaction, from the point at which theproduct reaches 100° C., does not exceed ten minutes. This makes it eveneasier to regulate the operation, as the time during which exothermicreactions may be initiated is reduced to a very brief period.

Advantageously, the torrefaction and the cooling are carried out in asingle chamber extending vertically and equipped with leak-proof devicesfor the introduction of the product at the upper part and for itsevacuation at the lower part, non-oxidizing gas at high temperaturebeing directed to the upper part under a pressure greater thanatmospheric pressure, --non-oxidizing gas at low temperature beingdirected to the lower part under a pressure greater than atmosphericpressure, a means for extraction being provided in order to move theproduct which is torrefied in the upper region of the chamber to thelower region, where it cools down. A compact unit in which heat lossesare reduced in thereby obtained. Additionally, the regulation of theoperation is facilitated: if, for example, following an overheating, theintroduction of hot gas is reduced, an afflux of cold gas whichamplifies the effect results therefrom in the central part, andtemperature lag is therefore reduced.

Advantageously, a part of the non-oxidizing gas evacuated from thecentral region of the chamber is redirected to the upper part of thechamber. This enables the atmosphere required at the upper part of thefurnace to be obtained, heat losses being restricted.

Also advantageously, drying precedes torrefaction without intermediatecooling and this drying is preferably carried out using hot gasesoriginating from the torrefaction. This is the drying at temperaturesbelow 100° C. The measure just mentioned restrict energy losses further.

The product of the invention is suitable for many applications.

In fact, it forms a high-grade raw material for a whole range ofchemical treatments, including combustion which will be discussed later.It should also be noted that the rot-proofness may be taken advantageof, to form, by mixing with concrete, light insulating agglomerates foruse in the building industry. It should also be noted that it may beadded to various wastes from forest plantations or from wood industrieswhich would otherwise be practically unusable, and which may thus beused easily, for example, in a burner.

The application of the product of the invention to energy productionwill now be described in greater detail.

Preferably, according to this application, almost all the pyroligneoussubstances contained in the torrefied product are extracted bydistillation, the remaining carbon is converted into carbon monoxide,the heat produced by this reaction being used for carrying out thedistillation, and the whole of the mixture of pyroligneous substancesand carbon monoxide is directed to a burner, the temperature of thepyroligneous substances being maintained above their dew point.

According to advantageous embodiments:

The distillation of pyroligneous substances and the conversion of carboninto carbon monoxide are carried out in the same hearth, which isprovided with one or more passages for gas in order to sweep away thepyroligneous substances on the one hand, and one or more passages forair, for the conversion of carbon into carbon monoxide on the otherhand, the hearth containing a common outlet for the pyroligneoussubstances distilled and for the carbon monoxide.

The gas intended for sweeping away the pyroligneous substances passes,in the direction of increasing temperature, through a mass of torrefiedproduct of sufficient thickness to ensure a substantially constantflowrate.

The hearth is of the vertical type, the torrefied product movingdownwards in this hearth, the sweeping gas passing downwards through themass of the torrefied product, the air inlet(s) for the conversion ofcarbon being located above a layer of ash maintained at a substantiallyconstant level, and the common outlet for the pyroligneous substancesand the carbon monoxide being located between the level of the ash andthat of the base of the distillation region.

An air induction type pilot burner capable of generating within thehearth a drop in pressure sufficient to suck in the sweeping gas and theair for the conversion of carbon is used.

The invention will now be described in greater detail using practicalexamples illustrated with accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the whole of the process;

FIG. 2 is a perspective view of a harvesting device in an operatingposition;

FIG. 3 is a diagrammatic cross-section of a torrefier;

FIG. 4 is a diagrammatic cross-section of equipment for the combustionof the product; and

FIG. 5 is a diagrammatic cross-section of a variant of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The diagram of FIG. 1 shows the following successive stages of theprocess:

- cultivation I - harvest II - cutting III - drying of the greensections IV - torrefaction V - application to energy production VIA orto chemistry VIB or in the building industry VIC or to the treatment ofcombustible wastes VID.

The process employed for the cultivation and the intensive production inan annual cycle, with mechanical harvest to the torrefiable woodysubstance in the form of vertical suckers or stems of small diameters,while aiming at increasing the vegetative period of the tress grown forthis purpose, comprises the implementation of a harvesting cut withoutdisturbing the collar of the tree on the one hand and determining theestablishment of a highly branched structure close to ground level onthe other, enabling the life of the trees to be extended and the harvestof the suckers to be mechanized.

It is known that the collar of the tree is the region (or the section)of the tree which separates, at the soil surface, the bundle of rootsfrom the aerial part consisting of the trunk and the branches. Anannular destruction of this collar by close cutting brings about adecline in health of the specimen within a short period of time.

Experience shows that the collar of the tree is saved if the cutting iscarried out above ground, even at a short distance of, for example, 10cm, provided that the portion of the stem remaining in the soil whichwould form the short trunk is no longer affected by the following cuts.

The process is also defined in that at the end of the year of planting,the first cutting of the rooted or transplanted specimens is carried outalong a plane equidistant from the soil, located some 10 cm above itssurface, the stems remaining in the soil being intended to form theshort trunks of the future trees.

The cuts which follow are more productive because the root bundles arewell established and many suckers will have formed.

According to the invention, close cutting are carried out when thebranches reach 1.5 to 2 centimeters in diameter at the base and 150 to200 centimeters in height, it being possible for their density to exceed100 stems per square meter.

The harvesting period in Europe is between November and February, infact, the leaves have dropped during this period; the shoots of the yearhave "lignified", i.e., changed into wood; the bark adheres to the woodof the stems, the amount of sap has decreased and the energy requiredfor drying will be lower.

Moreover, it is known that some species of trees such as fruit trees aresubject to annual prunings which leave on the branches the bases of theannular growths which most frequently have the shape of crooks. Thesecarry the buds which will be the points of the initiation of shoots inthe following year. This type of management which does not affect thelife of the tree enables the branches to be brought into shape.

The process according to the invention is also defined in that theannual harvest of the stems from the branches is carried out bymechanized cuttings along planes equidistant from the soil, but raisedby some 2 centimeters per year, so that the stem bases remaining on thebranches having the shape of crooks and carrying the buds which giverise to the shoots of he following year contribute to the expansion ofthe highly branched structure of the tree close to the ground.

Another feature of the invention relates to the distance betweenindividuals within a same row of trees. It must be such that theirexpansion which determines the survival of each tree is adequate.Meanwhile, the distance to be established between rows will be increasedby the space required for the free passage of the wheels of machineswhich must rest on the ground in the space between the rows, this spacebeing maintained by corrective prunings if required.

After the harvest, the open passage is used for carrying out theoperations relating to amendments, treatments or for loosening the soilor for hoeing it.

The invention also comprises a harvesting device representeddiagrammatically in the operating position in FIG. 2. The high densityof suckers to be harvested is shown at 1. The front-mounted cutter bar 2of the machine is of the same type as those used for the harvest ofcereals. The cut suckers 3 are directed towards the feeding rollers 4which regulate the speed of their movement towards the chopper drum 6containing the radial blades 7 rotating past the fixed blade 8. Thisshearing slices the suckers and divides them into small sections ofshort length 9. The latter are taken up by an elevator 10 and conveyedtowards the skip 11.

The surface 12 represents those left behind after the harvesting machinehas been in use.

A cross-section along a vertical plane located in the axis of a row oftrees after the harvest is shown at 14. The short trunks 15 and thehighly branched structure 16 of 2 trees are distinguished therein.

The passageway reopened, after the harvest, required for the freepassage of the wheels of the machine 17 is shown at 13.

The harvesting device according to the invention is defined in that itsuccessively straddles over the rows of trees and comprises afront-mounted cutter bar 2, the height of which can be adjusted,determines, on raising each year, that of the cutting plane above theground, the said bar making it possible, by its action, to harvest thesuckers of the year and similarly to reopen the passage 13 intended forthe wheels of the device, the passage previously occupied by thesuckers, facing each other and belonging to 2 consecutive rows of trees,the device simultaneously carrying out the slicing 6 of the suckers intosections 9 of short length collected in the skip 11.

By way of example, the height of the cutter bar 2 of the harvestingdevice, for the suckers of trees which have been in production for 20years, must be adjusted at 48 centimeters. In fact, to the 10centimeters (height of the first cut) should be added, 19×2 centimeters(value by which the height is raised annually), the result of whichoperation is indeed 48 centimeters.

The torrefiable woody product obtained exclusively consists of smallsections, resulting from the slicing of branches, roughly cylindrical,covered with their bark, the latter contributing to a large extent tothe quality and the appearance of the final fuel to be obtained andbeing a characteristic thereof.

The process employed for the cultivation of very different species oftree and shrubs with a view to harvesting the woody production thereofwill be advantageously applied in Europe or under similar climates tothe production of Laburnum which provides a high density of suckers andtakes up the nitrogen required by it from the atmospheric air, oralternatively of the apple tree or the pear tree which have theexceptional property of providing, after the torrefaction of thesections, a fuel with a bulk density of the order of 420 kg/m³.

The sections, as defined above, contain, depending on the nature of thewood they consist of, a relatively high quantity of water of between 40and 60%. This water should necessarily be eliminated before torrefactioncan be carried out.

The process according to the invention provides for an intensiveextraction of water still contained in the sections, the ducts of whichhave been opened at the two ends during the slicing, using a fast flowof high temperature gas sweeping their surfaces.

The process of roasting and torrefaction in a neutral atmosphere, offragments of wood, which are still moist, continuously brought in andcollected into a mass the height of which is kept constant, in avertically placed, truncated-shaped reactor, is defined in that, set inmotion by virtue of the low pressure created across the mass, theneutral heat-exchanging gas is raised to a temperature above thatrequired to achieve the torrefaction of the wood and sufficient to carryout, by roasting, an intensive drying out of the fragments as soon asthey are deposited in the reactor, drying resulting in a lowering of thetemperature of the gases which, mixed with the steam evolved constitutethe neutral gaseous flow and brought back to the temperature capable ofenduring, while it passes through it, the torrefaction of the mass.

It is a feature of the torrefaction method according to the invention tomake use of the water contained in the wood itself, converted intosteam, as one of the constituents of the neutral atmosphere in which thetorrefaction must be carried out.

In order to achieve the object of the process described above, it isindispensable that the heat exchanges between the heat-exchanging gasand the fragments of wood are rapid. This result is obtained by the useof a gas flow, surrounding the fragments, circulating at a high speedand with a temperature lag sufficient to ensure firstly the roastinggiving rise to the initial drying of the wood and secondly itstorrefaction.

It is also understood that it is economically advantageous to carry outthe operations of drying and torrefaction immediately in succession inthe same equipment.

The description which follows of a device, shown in diagrammaticcross-section in FIG. 3, according to the invention and the way in whichit operates will enable the process to be understood better.

The device comprises essentially an equipment called reactor 101 inwhich are carried out the operations of roasting producing the initialdrying, followed by torrefaction of the fragments of wood continuouslybrought in by the feeder 102. The fragments pass through the baffleplates 103, the purpose of which be described later, before beingdeposited on the roasting bed 104. This bed is maintained at constantheight by the action of a rotary plate extractor 105 which continuouslyremoves the wood once torrefied. The running speeds of the feeder andthe extractor are adjusted in a suitable ratio which takes into accountthe withdrawal which accompanies the drying and the torrefaction. Theresidence time in the reactor is determined by the transfer rate.

On extraction from the reactor, the torrefied wood is deposited at thetop 106 of the mass collected in the receptacle 107. This latter, of acertain capacity is equipped at its base with a suitable double-locksystem, not shown, which enables the torrefied wood to be withdrawn insuccessive baskets, without allowing air to penetrate into the device.

A high pressure fan 108 lowers the pressure in the device comprising thereceptacle, the extractor and the reactor. This low pressure is conveyedto the device through the condenser-exchanger 109, the pipe 110, theannular nozzle 111 and the opening 112. It is applied to the mass 115 inthe course of the treatment passing through the rotary and perforatedblade 105 at the base of the skirt of the reactor 113 which is alsoperforated and the annular space 114 occupied by fragments of torrefiedwood during the transfer.

The large surface area that these parts located at the base of thereactor offer to the passage of the gaseous flow should be noted. Thisarrangement slows down the flow-rate the brisk force of which is nolonger sufficient to carry away the fragments located in the space 114.

This low pressure sets in motion the flow of the heat-exchanging gascirculating at high speed between the fragments of wood deposited on thebed 104, flow used for successive treatments of roasting andtorrefaction of the wood.

The process according to the invention is also defined in that theheat-exchanging gas consists of a mixture of a large part of recycledneutral gases taken up from the outlet of the reactor 101, and which aretherefore at the torrefaction temperature, and burnt gases resultingfrom the combustion in the chamber 118 of a combustible gas supplied tothe burner 116, the temperature of which mixture, being two to threetimes greater than that applied during the torrefaction which willfollow, is capable of carrying out on the fixed bed 104 of the reactor101 the initial roasting of the moist wood.

The recycled gas is taken up into the circuit at 120 with the aid of theblower 121. The combustible gas is brought to the annular burner 116through the pipe 117.

The blower 121 brings back the gas taken up at the outlet of the reactorat a pressure sufficient to feed the reactor 101 and to create acrossthe baffle plate 103 a slight increase in pressure against the entry ofair at this level, but allowing the entry of the fragments of woodbrought by the feeder 102.

The combustible gas will advantageously be that obtained from a portionof the torrefied wood itself, taken up from the production and gasified.The order of magnitude of the amount consumed is approximately 10% to12%.

The time required for the successive operations of roasting giving riseto the intensive drying of the wood and its torrefaction settles downbetween 3 and 6 minutes depending on the temperatures employed and themeans size of the fragments (less than or equal to 1 cm³). These timeswhich are given by way of example are not limiting. They correspond tothe mean residence time of the fragments in the reactor.

The gas from the reactor which has not been recycled passes through theexchanger 109 before being released into the atmosphere. This exchangeris also a condenser, it enables the water collected at 127 and evacuatedwith a pump, which is not shown, to be extracted and the caloriescontained in the gas to be recovered. These calories are transferred toambient air brought to the exchanger 109 by the fan 125 and driven,passing through the pipe 126, towards a wood drier, not shown, installedupstream of the reactor. The purpose of this drier is to decrease byapproximately 50% the water content of the wood, which is the rawmaterial and to preheat it to a temperature in the vicinity of 90° C.These techniques are sufficiently known so that they need not bedescribed in detail.

The process according to the invention is also defined in that a part ofthe neutral gases, cooled on passing through the exchanger 109 andintroduced by the annular nozzle 124 at the base of the receptacle 107is set in motion by the low pressure prevailing in the device and passesthrough the mass 129 of torrefied wood in order to extract therefrom therecoverable calories and to reduce the temperature therein so as toprevent the possible spontaneous ignition during the exposure to theatmosphere of the wood at the torrefaction temperature.

In view of the temperatures employed, the whole of the device is coveredwith a heat insulator 128.

As it is brought to high temperature, the combustion chamber 118 iscovered with a fire-proof cover 130, the purpose of this chamber beingto ensure the complete combustion of the combustible mixture which isachieved without an excess of air. In fact, the gaseous flow must beneutral.

The upper part of the reactor forms part of the combustion chamber. Withits internal radius, it contributes to the homogeneity of temperatureover the entire section of the heat-exchanging flow.

The neutral gases, cooled by the exchanger 109 and extracted by the fan108 contain a certain proportion of smokes consisting of tars formedespecially during the roasting carried out in the reactor 101.

In order to prevent these smokes from being wasted and to reintegrateinto the fuel and in this form one of the component parts, the processis finally defined in that the gases are filtered by passing through alarge mass of wood already torrefied and collected in a transfer silooperating continuously before the storage of the same fuel.

The tars deposited in very small amounts at the surface of the fragmentseasily diffuse into the wood which has become porous after thetorrefaction.

The arrangements made for the determination of pressures, flowrates andtemperatures and also for monitoring the operation of the device departfrom the scope of the invention and have therefore not been described.

The torrefaction carried at 250°-280° C. in an equipment as justdescribed, results in its water content being fixed at 3% and produces aproduct with stable characteristics therefrom. Besides, it is known thattorrefaction does not remove from the wood pyroligneous products whichrepresent approximately 50% of the total energy potential. The torrefiedwood has a calorific value of the order of 5,200 kcal/Kg. It lendsitself to mechanised handling without forming dusts. It keeps well evenin wet premises. It ignites easily. Pyroligneous vapors may be extractedbetween 250° and 350° C., tars at about 350° to 500° C., and charcoalremains beyond this temperature.

Finally, it is known that in the technology of the so-called inductiongas burners, use is made of axial (venturi) or annular nozzle tuyeres,supplied with the so-called inductor fluid under pressure whereasupstream of the nozzle the resultant low pressure sets in motion theso-called induced fluid. The pilot and inductor fluid may be fuel gas orair which may be previously heated.

In the combustion device which will be described the initial fuel thatthe torrefied wood represents is converted into a mixture ofpyroligneous vapors and carbon monoxide intended to be supplied to aburner capable of providing a continuous flame which may be used inthermal equipment which exist in substitution for oil or gas burners.

The combustion device, which consists of firstly, the hearth, the mainobject of which is to enable pyroligneous vapors to be extracted, thisoperation being carried out by virtue of the calories originating fromthe partial combustion of charcoal, which is the residue of theextraction, the partial combustion additionally providing a certainquantity of carbon monoxide, and secondly, the burner, the object ofwhich is to ensure the complete combustion of the combustible mixturereduced in the hearth, this device being defined in that the hearth andburner are combining into a single unit so that the temperature of thevapors and of the pipe for the transfer cannot be lowered below thecondensation temperature of the heaviest fraction distilled.

The combustion device may cover two embodiments: in the first, thehearth and the burner are coupled, and the tuyere tube itself forms theregion raised to a high temperature where the flame front settles down;in the second, the hearth and the burner are separated by a pipe, whichis thermally isolated and contains the tuyere, in which the inflammablemixture flows at a speed greater than that of the flame propagation,this latter settling down and intensifying in the truncated combustionchamber located at the end of the pipe. The burner may be litelectrically as known per se.

The supply of torrefied wood to the device is carried out mechanicallyand is aimed at keeping filled a tubular column open at the top locatedabove the hearth so that the height occupied by the torrefied wood inthe latter and the pressure loss of an airflow passing through itdownwards are constant.

The supply of pilot air which ensures the combustion of the vapors andof the combustible gases in the tuyere, creates upstream a low pressurewhich enables the combustion air to be introduced at the level of thehearth on the one hand the pyroligneous vapors to be swept downwards andthrough the torrefied wood supply column, on the other.

According to the invention, the hearth consisting of embers resultingfrom the partial combustion of charcoal rests on a floor made up of thebed of ash itself. The stable position of the bed in the device isobtained by the use of a mechanical extractor.

FIG. 4 represents the device resting on a bracket 201 in the vicinity ofthe peephole of a thermal chamber to be heated 202.

The screw 203 intended for the mechanical extraction of ash, above whichthe bed of ash 204 on which the ember region 205 surmounted by thereserve of torrefied wood 206 filling the removable pipe 207 rests islocated is shown in the base of the device in diagrammaticcross-section.

At the level of the ember region, that is 10 to 20 cm above the ash,holes 208 through which the air for the combustion of charcoal isintroduced, are made in the wall of the body of the device.

The inner chamber of the device then develops towards the tuyere 209.The slope for the fall-back 215 formed by the glowing embers is seen atthe base of the connecting pipe 210 inclined upwards.

The annular nozzle 211 communicating with the chamber 212 and the pipe213 for the supply of pilot air at air for the combustion of vapors andgas in the burner are located at the constriction of the tuyere.

The diverging part 214 of the tuyere has a large diameter as its formsthe combustion chamber itself, in which the flame front settles down inthe burner.

A low pressure resulting from the very principle of operation of thetuyere is created upstream of the burner, i.e., in the hearth itself.

The starting up and operation of the device may be carried out asfollows: a batch of ash is used to form the initial floor. Charcoal isdeposited on the latter up to the level of the fall-back slope 215. Thepilot air pressure is set up in the chamber 212. The flow of air in thenozzle 211 creates a low pressure in the hearth. After removing theremovable pipe 207, the charcoal placed on the bed of ash is ignitedwith a flame and the tube 207 is then replaced in its position andfilled with torrefied wood. Almost immediately, the device emits a densewhite smoke leaving the tuyere 214, being pyroligneous vapors, which maythen be ignited. An electric device, not shown, may be used for thispurpose.

As a result of the low pressure created in the hearth, air penetratesthrough the holes 208 provided for this purpose and maintains thecombustion of the charcoal. The same low pressure draws in a weakcurrent of air circulating downwards in the tube 207 passing through thetorrefied wood. This air flow sweeps the pyroligneous vapors extractedat the approach of the hearth, drives them towards the supply pipe tothe burner, where they mix with the very hot carbon monoxide in thehearth and then, finally, with the preheated pilot air supplied to theburner to form in the latter the flame front which will develop in thethermal chamber 207.

FIG. 5 represents a variant of the device in which the injector tube ofthe tuyere 217, of the venturi type, is much more elongated. Thecreation in the pipe 218 of a flowrate of the inflammable mixturegreater than that of the propagation of the flame which will then be setup in a second injector tube of larger diameter 219, located at the endof the pipe 218, results therefrom. This arrangement makes it possibleto supply a furnace in the proximity of which it is impossible to placethe device of FIG. 4 satisfactorily.

The pipe 218 must be suitably insulated so as not to allow a lowering oftemperature of the pyroligneous vapors, and the for the same reason, theair for the combustion introduced to the axial injector 220 must beadequately preheated.

It should be noted that the geometric configurations of the product ofthe invention ensures its regular descent towards the hot region of thehearth, and a constant mass permeability for the sweeping air. A highsteadiness of temperature distribution, and consequently of thecomposition of the combustible mixture supplied to the burner resultstherefrom.

I claim:
 1. A process for producing torrefied wood products which can beburned to produce thermal energy from plants which produce substantiallystraight, unbarked woody suckers having diameters of 5 to 20 mm, saidprocess comprising the steps of(a) cutting said substantially straight,unbarked woody suckers having diameters of 5 to 20 mm from said plants,(b) sectioning said substantially straight, unbarked woody suckers so asto produce unbarked green sticks of a uniform length, said uniformlength being between about 10 and 25 mm, (c) placing said unbarked greensticks provided in step (b) in a reactor so as to form a bed therein,(d) passing a hot, non-oxidizing gas having a temperature greater than280° C. through said bed so as to first dry and then torrify theunbarked green sticks therein, thus providing torrefied wood products,the velocity of said hot, non-oxidizing gas being sufficiently low toavoid conveying of torrefied wood products away from said bed yetsufficiently high to raise the temperature of the unbarked green sticksfrom 100° C. to a torrification temperature of 250° to 280° C. in nomore than 10 minutes, and (e) cooling said torrefied wood products. 2.The process according to claim 1, wherein in step (a) the substantiallystraight, unbarked woody suckers are cut from said plants at a levelabove a collar thereof.
 3. The process according to claim 1, wherein thesubstantially straight, unbarked woody suckers cut from said plants instep (a) have a length of 1.5 to 2 meters.
 4. The process according toclaim 1, wherein the substantially straight, unbarked woody suckers cutfrom said plants in step (a) have no leaves thereon.
 5. The processaccording to claim 1, wherein in step (b) said substantially straight,unbarked woody suckers are aligned in parallel and then cut by at leastone rotating cutting blade.
 6. The process according to claim 1, whereinsaid hot, non-oxidizing gas consists essentially of a mixture of watervapor and combustion gas.
 7. The process according to claim 6, wherein afirst portion of said hot, non-oxidizing gas, after passing through saidbed, is recirculated so as to again pass through said bed.
 8. Theprocess according to claim 7, wherein between steps (d) and (e) saidtorrefied wood products are removed from said bed and accumulated in amass and wherein in step (e) a cool, non-oxidizing gas is passed throughsaid mass.
 9. The process according to claim 8, wherein a second portionof said hot, non-oxidizing gas, after passing through said bed, iscooled to provide said cool, non-oxidizing gas.
 10. The processaccording to claim 6, wherein said hot, non-oxidizing gas is passeddownwardly through said bed.
 11. The process according to claim 9,wherein said cool, non-oxidizing gas is passed upwardly through saidmass.
 12. The process according to claim 1, wherein said plants in step(a) are laburnum.
 13. The process according to claim 1, wherein saidplants in step (a) are apple trees.
 14. The process according to claim1, wherein said plants in step (a) are pear trees.
 15. Torrefied,unbarked wood products which are generally cylindrical in shape andwhich have diameters between 5 and 20 mm and uniform lengths of about 10and 25 mm, said torrefied, unbarked wood products being made by thesteps of(a) cutting substantially straight, unbarked woody suckershaving diameters of 5 to 20 mm from plants, (b) sectioning saidsubstantially straight, unbarked woody suckers so as to produce unbarkedgreen sticks of a uniform length, said uniform length being betweenabout 10 and 25 mm, (c) placing said unbarked green sticks provided instep (b) in a reactor so as to form a bed therein, (d) passing a hot,non-oxidizing gas having a temperature greater than 280° C. through saidbed so as to first dry and then torrify the unbarked green stickstherein, thus providing torrefied wood products, the velocity of saidhot, non-oxidizing gas being sufficiently low to avoid conveying oftorrefied wood products away from said bed yet sufficiently high toraise the temperature of the unbarked green sticks from 100° C. to atorrification temperature of 250° to 280° C. in no more than 10 minutes,and (e) cooling said torrefied wood products.
 16. Torrefied woodproducts according to claim 15, wherein said plants in step (a) arelaburnum.
 17. Torrefied wood products according to claim 15, whereinsaid plants in step (a) are applied trees.
 18. Torrefied wood productsaccording to claim 15, wherein said plants in step (a) are pear trees